1. Field of the Invention
This invention relates to the exposure control of an image pickup apparatus.
2. Description of the Related Art
A method of controlling the aperture of an iris, the storage time of an image sensor (electronic shutter) and the gain of an AGC (automatic gain control) circuit for keeping the level of video signals at a predetermined value is employed in general for exposure control in image pickup apparatuses such as video cameras or the like. An image pickup apparatus of this kind is arranged and operates as described below with reference to FIG. 1 which is a block diagram.
Referring to FIG. 1, a zoom lens 1 which has a variable focal length as a lens group is arranged to form the image of an object of shooting. An iris mechanism 2 is arranged either to be of an iris blade structure to control the quantity of incident light or to be in the form of liquid crystal means to control the quantity of transmission light. In this case, an iris is employed. An image sensor 3 is arranged to photo-electrically convert the incident light. A driving motor 6 is an IG meter or the like arranged to drive the iris mechanism 2. An image sensor control circuit 7 is arranged to control the image sensor 3 in such a way as to read out a photo-electrically converted signal and to control the signal storage time in a manner called an electronic shutter function.
A sample-and-hold (S/H) circuit 8 is arranged to sample the signal photo-electrically converted by the image sensor 3. An automatic gain control circuit (hereinafter referred to as AGC circuit) 9 is arranged to electrically amplify a signal. A camera signal processing circuit 10 is arranged to form a standard television signal by carrying out a gamma correcting process, a color separating process, a color-difference matrixing process, etc., and, after that, by adding synchronizing signals. The camera signal processing circuit 10 may be either an analog signal processing circuit which processes a signal in the form of an analog signal or a digital signal processing circuit which is arranged to analog-to-digital convert a video signal, to process the digital-converted signal and, after that, to digital-to-analog convert the signal. In this case, however, the camera signal processing circuit is an analog signal processing circuit.
A video tape recorder (hereinafter referred to as VTR) 11 is arranged to record a video signal on a tape. An electronic viewfinder (hereinafter referred to as EVF) 11 is provided for monitoring video images being picked up.
An exposure detecting circuit 13 is arranged to detect the state of exposure for the shooting object through a signal outputted from the AGC circuit 9. An exposure control circuit 22 is arranged to control the iris mechanism 2, the shutter speed of an electronic shutter which is controlled by the image sensor control circuit 7, the gain of the AGC circuit 9, etc., on the basis of a detection signal outputted from the exposure detecting circuit 13. An iris driving circuit 16 is arranged to operate, according to the output of the exposure control circuit 22, the motor 6 which drives the above-stated iris mechanism 2. A trigger key 14 is provided for inputting a trigger signal to perform an AE lock action by which an exposure control state is maintained.
The exposure control circuit 22 contains therein an exposure control computing part 22a which is arranged to compute control information for control over the AGC gain, the shutter speed of the electronic shutter and the amount of aperture of the iris according to the signal outputted from the exposure detecting circuit 13 indicating a state of exposure, such as an integrated value of luminance level, a data holding part 22h composed of an AGC data holding part 22b, an electronic shutter data holding part 22c and an iris data holding part 22d which are arranged respectively to hold, in response to the operation of the trigger key 14, control information on the AGC gain, the shutter speed of the electronic shutter and the amount of aperture of the iris outputted from the exposure control computing part 22a, and an AGC control part 22e, an electronic shutter control part 22f and an iris control part 22g which are arranged respectively to control the gain of the AGC circuit 9, the speed of the electronic shutter of the image sensor 3 and the amount of aperture of the iris mechanism 2 according to data of varied kinds supplied through the AGC data holding part 22b, the electronic shutter data holding part 22c and the iris data holding part 22d. 
In order to obtain optimum video images by a simple shooting operation under various conditions, in the image pickup apparatus, the exposure detecting circuit 13 is arranged to detect changes in luminance of the object of shooting from a video signal, and the exposure control circuit 22 is arranged to select and correct exposure control parameters such as the aperture of the iris mechanism 2, the electronic shutter controlling the storage time of the image sensor 3, the gain of the AGC circuit 9, etc., on the basis of the detection signal from the exposure detecting circuit 13. The image pickup apparatus is thus arranged to perform control for making an optimum exposure always in a stable manner. Such an arrangement enables the automatic exposure control means to automatically carry out optimum exposure control, without recourse to any manual operation by the operator, even when the luminance of the object varies.
However, according to the above-stated automatic exposure control means, since the state of exposure is controlled always according to the luminance of the object image incident thereon through the zoom lens 1, the state of exposure control varies when the luminance of objects located around the main object of shooting happens to change due to a change taking place in focal length of the zoom lens 1, a motion of the main object or the like, even if the luminance of the main object does not vary.
Therefore, if the luminance of peripheral objects changes even when the state of exposure is in an optimum state for the main object of shooting, the state of control over the exposure comes to change and eventually becomes inapposite to the main object.
To cope with such a shooting condition, a means for holding a state of exposure called xe2x80x9cAE lock meansxe2x80x9d have been developed to hold the apparatus in an apposite state of exposure control. The AE lock means is operated as follows. The operator watches the state of exposure on the image plane of the EVF 12 which is a monitoring means and inputs a trigger signal from the trigger key 14 when the operator wishes to keep the current state of exposure unvarying. In response to the trigger signal input, the data holding part 22h of the exposure control circuit 22 is brought into a holding state, in which amounts of correction computed by the exposure control computing part 22a at that time for correction of the exposure control parameters of varied kinds including the aperture of the iris mechanism 2, the speed of the electronic shutter controlling the storage time of the image sensor 3 and the gain of the AGC circuit 9 are held.
In a case where a difference in luminance between the main object and peripheral objects are too much to obtain an optimum state of exposure for the main object by the automatic exposure control means, like in shooting a person under a back-light condition or like in shooting a person illuminated with an excessive front spot light, the exposure state for the person, i.e., the main object, becomes too dark due to an adverse effect of a bright background to result in so-called xe2x80x9cblack blankingxe2x80x9d or too bright to result in so-called xe2x80x9cwhite blankingxe2x80x9d. Under such shooting conditions, the AE lock means is sometimes used to eliminate the adverse effect of the luminance of the peripheral objects by varying the focal length of the zoom lens 1 to zoom up the main object, performing an AE lock action when the exposure state for the main object is optimized, and, after that, shifting the focal length of the zoom lens 1 back to the original focal length so that the shooting operation is allowed to begin by adjusting the video image to the composition of picture desired by the operator.
The use of the AE lock means, however, still leaves a problem unsolved in the following point. Even when the quantity of incident light from the zoom lens 1 is unvarying and the iris mechanism 2 which limits the quantity of incident light remains in a constant state, the F number of the zoom lens 1 changes according to its focal length. The change of the F number then causes the quantity of light incident on the surface of the image sensor 3 to vary. Therefore, even if the AE lock action is performed in an optimum manner, the state of exposure obtained by the AE lock action comes to vary when the focal length of the zoom lens 1 is varied, although the state of control over the exposure parameters of the iris mechanism, the AGC circuit, the electronic shutter, etc., remains unchanged. Under such a condition, it is hardly possible to maintain an optimum state of exposure.
This invention is directed to the provision of an exposure control device and an image pickup apparatus which are advantageously adapted for a video camera having a zooming function.
It is, therefore, an object of this invention to provide an image pickup apparatus arranged to include AE lock means and yet to be capable of retaining an optimum state of exposure obtained when an AE lock action is performed, even if the focal length of a zoom lens is varied.
To attain this object, an image pickup apparatus arranged as a preferred embodiment of this invention comprises a zoom lens having a variable focal length and serving as a lens group for forming an image of an object, an image sensor for photo-electrically converting light incident through the zoom lens into a signal, signal processing means for processing the signal photo-electrically converted by the image sensor to form a video signal as a television signal, exposure control means for controlling a state of exposure according to the video signal, exposure state holding means for holding the state of exposure controlled by the exposure control means, focal length detecting means for detecting the focal length of the zoom lens, and exposure correcting means for correcting the state of exposure controlled by the exposure control means and held by the exposure state holding means, according to the focal length detected by the focal length detecting means.
According to another preferred embodiment of this invention, the signal processing means includes automatic gain control means for electrically amplifying the video signal, and the exposure correcting means is arranged to correct the state of exposure by controlling the gain of the automatic gain control means according to a change of the focal length of the zoom lens.
According to a further preferred embodiment of this invention, the exposure control means has an electronic shutter function to control a storage time of the image sensor, and the exposure correcting means is arranged to correct the state of exposure by controlling the storage time of the image sensor according to a change of the focal length of the zoom lens.
An image pickup apparatus arranged as a still camera further preferred embodiment of this invention comprises a zoom lens having a variable focal length and serving as a lens group for forming an image of an object, an image sensor for photo-electrically converting light incident through the zoom lens into a signal, iris means for limiting a quantity of the light incident on the image sensor, aperture state detecting means for detecting an aperture state of the iris means, signal processing means for processing the signal photo-electrically converted by the image sensor to form a video signal as a television signal, exposure control means for controlling a state of exposure according to the video signal, exposure state holding means for holding the state of exposure controlled by the exposure control means, focal length detecting means for detecting the focal length of the zoom lens, and exposure correcting means for correcting the state of exposure controlled by the exposure control means according to the focal length detected by the focal length detecting means and the aperture state detected by the aperture state detecting means.
It is a further object of this invention to provide an image pickup apparatus arranged such that, in a case where a zooming action is performed in an AE lock state in which an exposure state is maintained by the exposure state holding means, a change in exposure caused by a change in F number resulting from a change in the focal length of the zoom lens is corrected by detecting the change of the focal length and by correcting the state of exposure controlled by the exposure control means and held by the exposure state holding means.